1. Field of the Invention
The invention relates to a motor with a frequency generator attached thereto, equipped with the frequency generator (FG) for detecting a rotational speed of the motor, that is, a motor with a frequency generator for driving drums, used in office automation (OA) equipment such as, for example, a laser beam printer (LBP), and OA equipment using the same.
2. Description of the Related Art
A conventional motor with a frequency generator attached thereto comprises a magnet in a ring-like shape, having 10 poles of main magnetic poles for driving the motor, provided at equal pitches in the radial direction, and a plurality of frequency generator magnetic poles, provided at equal pitches on an end face of the magnet, in the direction of a motor axle, and rotating integrally with the motor axle, a stator core having 12 salient poles of main magnetic poles, disposed opposite to the inner circumference of the magnet with a gap interposed therebetween and a printed wiring board disposed so as to oppose the end face of the magnet with a gap interposed therebetween, and provided with coil patterns of the frequency generator, formed thereon.
With the motor with the frequency generator attached thereto, having a configuration as described above, the frequency generator magnetic poles, provided at the end face of the magnet, cut across power generation wire-elements of the coil patterns, thereby generating a frequency generator signal.
However, because the magnet has radial anisotropy, a magnetic force emanating from the end face thereof, in the direction of the motor axle, is weak. Accordingly, if a rotational speed of the motor is low, an output voltage of the frequency generator drops, causing a problem that the output voltage cannot be recognized as the frequency generator signal. Further, main magnetic pole components of the magnetic force are superimposed on the output voltage of the frequency generator to thereby cause distortion to a waveform of the frequency generator signal as generated, so that the frequency generator signal deteriorates in accuracy, adversely affecting variation in rotational speed, and so forth.
Further, if a method is adopted whereby a magnet having thrust anisotropy is fixedly attached to the end face of the magnet having radial anisotropy in order to overcome the problem described, this will cause reduction in the main magnetic pole components of the magnetic force, resulting in improvement of an S/N ratio of the frequency generator. However, this will cause a problem of an increase in cost due to an increase in the number of components.